The present invention is directed to optical components for use in fiber optic networks and particularly to devices known as optical polarization splitters (dividers) or combiners (multiplexers).
In fiber optical transmission systems the light beams traveling in two fibers must often be combined into a single fiber, a device which accomplishes this is called combiner or multiplexer. Similarly, in such systems one beam must frequently be split into two or more beams, a device which accomplishes this is called a splitter or divider. A splitter or combiner is simply the same device used xe2x80x9cin reversexe2x80x9d i.e. when a beam is launched from a single fiber through a splitter/combiner the beam will be split into two beams and directed to two output fibers, if two beams are launched from the previous xe2x80x9coutputxe2x80x9d (now input) fibers back through the splitter/combiner the beams will be combined into a single beams and directed to the single xe2x80x9cinputxe2x80x9d (now output) fiber. However the use of standard splitter/combiners brings with it an important consideration: each splitting or combining of the beam causes a 3 dB loss of light power, as a beam traveling through a fiber optic transmission system may undergo many splittings and combinations the cumulative effect of the 3 dB losses can be very large.
One practicable splitter/combiner which does not incur the 3 dB losses is a so called polarization splitter/combiner in which an incoming beam having two orthogonal linear polarizations is split into two beams by being passed through a displacer or by the use of a polarizing beam splitter (either a prism cube or a thin film on a glass plate) with one beam having a first linear polarization state and the other a second linear polarization state with the first and second polarization states being orthogonal to each other. When used as a combiner a beam of a first linear polarization state from one input fiber is combined with a beam of second linear polarization state from a second input fiber into a third output fiber carrying both beams.
Previous polarization splitter/combiners relying on the use of polarizing beam splitters and birefringent displacers have required three lenses to couple light into the input and output fibers. In addition, in the case of polarizing beam splitter or film, the polarization extinction ratio is limited to 20-30 dB. The present invention is directed to an optical polarization splitter/combiner of the xe2x80x9cinlinexe2x80x9d type which provides a more compact splitter/combiner by displacing the beams in angle instead of position. This has the advantage of using one collimating lens per two input or output fibers with all of the input and output fibers lying parallel to each other. The reduction in component count also greatly simplifies the necessary alignment of the components and thus reduces cost. The present invention also provides for the ready alignment of the components of the splitter/combiner to reduce insertion loss thus further saving cost and complexity.
In the polarization based optical splitter/combiner of this invention, two birefringent wedges are aligned with their optical axes at 90xc2x0 from each other, with the optical axes perpendicular to the direction of light propagation, and with the base of one wedge disposed upwardly and the base of the other wedge disposed downwardly. When used as a splitter, one lens is used at the input to couple light from the input fiber (thus sending a collimated beam into the two birefringent wedges). The birefringent wedges will split the incoming beam into two component orthogonal polarization states, one beam having an E (extraordinary) polarization state and the other an O (ordinary) polarization state, with respect to the considered birefringent wedge. A single output lens is used to couple the output beams into two adjacent parallel polarization maintaining (PM) or single mode (SM) fibers, the PM fibers can have any optic axis orientation which is convenient. When used as a combiner an E beam from a first fiber is combined with a O beam from a second fiber into an single output beam. The common port fiber (the input fiber in a polarization divider operation or the output fiber in polarization combiner operation) can be a polarization maintaining fiber or a single mode fiber (non-polarization maintaining fiber) depending on the application requirement.